This invention relates to photoflash lamps and, more particularly, to flashlamps of the type containing a primer arrangement ignited by a high voltage pulse.
High voltage flashlamps may be divided historically into four categories: (1) those having a spark gap within the lamp such that electrical breakdown of a gaseous dielectric (e.g., the combustion-supporting oxygen atmosphere) is an integral part of the lamp ignition mechanism; (2) those having spaced apart electrodes, at least one of which is coated with a primer material, and relying on shreds of combustible metal foil in the lamp to provide a conducting path between the electrodes; e.g. see Albrecht U.S. Pat. No. 2,868,003; (3) those having a conductive primer bridge that electrically completes the circuit between the lead-in wires; such primers are rendered conductive by additives such as acetylene black, lead dioxide, or other electrical conduction-promoting agents; and (4) lamps having an essentially non-conducting primer bridge that connects the inner ends of the lead-in wires and which becomes conductive, upon application of a high voltage pulse, by means of breakdown of the dielectric binder separating conductive particles therein.
The earliest high voltage flashlamps were of the spark gap type construction wherein an electrical spark would pass through the gaseous atmosphere within the lamp. The spark would jump between two electrodes, at least one of which was coated with a primer composition. Such lamps tend to exhibit poor sensitivity and reliability when flashed from low power sources such as the miniaturized piezoelectric devices that are suited for incorporating into pocket-sized cameras. Most of the electrical input energy in such lamps is lost to the gap atmosphere by the spark. Also, the electrical characteristics vary considerably from one lamp to another because of shreds of metallic combustible in the spark gap and consequent variations in effective gap length.
In lamps of the second category, shreds of metal fill material are in contact with both the primer on one electrode and with the other electrode and form an electrically conducting path therebetween, such that, upon application of high voltage current to the electrodes, a spark discharge is formed through the primer material via the shreds and electrodes. In this manner, the above-noted disadvantages of the somewhat similar looking spark gap lamp are avoided.
The use of spaced lead-in wires interconnected by a quantity of electrically conductive primer gives rise to highly predictable behavior and a well-defined electrical path through the lamp. Here again, however, relatively highpowdered flash sources must be used in order to attain reliable lamp flashing.
Present state of the art flashlamps of the high voltage type make use of a bridge of initially nonconducting primer to interconnect the inner ends of the lead-in wires. Considerably higher sensitivity is attainable by this method, apparently because the breakdown and discharge follow a discrete path through the primer composition and thereby promote greater localized heating. With respect to specific construction, such flashlamps typically comprise a tubular glass envelope constricted and tipped off at one end and closed at the other end by a press seal. A pair of lead-in wires pass through the glass press and terminate in an ignition structure including a glass bead, one or more glass sleeves, or a glass reservoir of some type. A mass of primer material contained on the bead, sleeve or reservoir bridges across and contacts the ends of the lead-in wires. Also disposed within the lamp envelope is a quantity of filamentary metallic combustible, such as zirconium, or hafnium, and a combustion-supporting gas, such as oxygen, at an initial fill pressure of several atmospheres.
Lamp functioning is initiated by application of a high voltage pulse (e.g., several hundred to several thousand volts, as for example, from a piezoelectric crystal) across the lamp lead-in wires. The mass of primer within the lamp then breaks down electrically and ignites; its deflagration, in turn, ignites the shredded combustible which burns actinically.
The fabrication and testing of a number of different ignition structures has shown several problem areas that are peculiar to high voltage type flashlamps, and which are familiar to those skilled in the art of flashlamp design. For example, random location of the shreds of metallic combustible can cause short circuiting of the lead-in wires or interfere with the intended electrical breakdown path through the primer. Post-flash short circuiting can be caused by primer residue, metallic or semimetallic droplets of slag from the ignited shreds of combustible, and possible welding of the lead-in wires after ignition.
A further problem deals with the primer material itself. Previous primers for high voltage flashlamps have comprised a combination of at least three types of ingredients: (1) a combustible metal powder, such as zirconium (some applications have also included magnesium as a conduction-promoting agent); (2) an oxidizer salt, such as potassium chlorate or perchlorate; and (3) a binding agent such as nitrocellulose or polyvinyl alcohol. A fourth ingredient, called a sensitizer, may be used in addition to the first three. The sensitizers, may comprise either one or more of three distinct types taught; (a) fuels with very low ignition temperature, such as red phosphorus, sulfur, or selenium; (b) powdered semiconductors which modify the electrical conductivity of the primer bridge before or during electrical discharge therethrough, and represented by PbO.sub.2, MnO.sub.2, CuO, NiO, and LaCoO.sub.3 doped with, e.g., Sr; and (c) organic nitrocompounds, such as tetrazene and lead styphnate.
High voltage primers of the above-described types comprising Zr powder (alone or with Mg powder), an oxidizer salt such as KClO.sub.4, and a binder such as nitrocellulose, (either with or without "sensitizers") may be rendered quite sensitive toward ignition from low energy high voltage pulses. It is found that electrical input energy for reliable ignition is greatly dependent upon the amount of oxidizer salt present; as the relative amounts of, e.g., KClO.sub.4 in the formulation increases, the required electrical triggering energy decreases. High loadings of such oxidizer salts (for example, 10-40% by weight of the dried primer) are thus favored.
Two real problems are inherent in all such high oxidizer salt primers, however. One is the hazardous nature of such mixtures to prepare and handle in production quantities. The other problem involves the quantity of smoke generated during deflagration of such mixtures. Studies have shown the smoke to comprise volatile chlorides of the alkali or alkaline earth metals used as the oxidizer salt. For example, when KClO.sub.4 is decomposed to give up its oxygen content in a hot pyrotechnic mixture, KCl vapor is formed and is often the major component of the resulting white smoke.
The use of red phosphorus or sulfur as low-igniting sensitizers similarly has been found to cause formation of dense white smoke and deposits on the wall of the lamp vessel. In these cases, it is the P.sub.2 O.sub.5 and SO.sub.3 formed by burning of these elements in the oxygen rich atmosphere of the flashlamp.
Photometric tests have shown that any such smoke generation in a flashlamp as a result of primer deflagration seriously reduces the photographically useful light output from such a lamp.
Yet another problem associated with oxidizer salt primers has been encountered in arrays of high voltage flashlamps employing such primers. The lamps are intended to be selectively fired one at a time; however, undesired simultaneous firing of two or more lamps at a time has occurred, apparently due to leakage of the high voltage in the array structure and circuitry.